Reproduction, Sexual
Sexual reproduction is the creation of new individuals resulting from the joining of the nuclei of two separate sex cells. The separate sex cells are gametes; the sperm and egg. Fertilization is the union of the two gametes, and the fertilized cell that is formed is called the zygote.
These gametes must have specific characteristics in order for sexual reproduction to work properly. Specifically, they must have only half the total number of chromosomes that normal cells of that species contain. Cells that have half the normal number of chromosomes are called monoploid or haploid cells. Thus, when the nuclei of the sperm and egg join together, instead of having twice the normal number of chromosomes, the resulting fertilized egg cell will have exactly the right number. A cell with the normal number of chromosomes is called a diploid cell. In order to form a monoploid sperm or egg cell, specific reproductive cells undergo a special type of cell division called meiosis. The sperm or egg cells that are formed as a result of meiosis are ready to be used as part of sexual reproduction. Meiosis forms the sex cells of all organisms that undergo sexual reproduction.
Sexual reproduction is very different from asexual reproduction, in which there is only one parent. In asexual reproduction, there are no gametes, and hence no fusion of the sperm and egg. Because there is only one parent, the offspring produced by asexual reproduction are genetically identical to the parent. This gives sexual reproduction a great advantage over asexual reproduction. Sexual reproduction produces variation among the offspring; the offspring are not exactly like the parents. If the environment changes, some individuals may be better able to survive than others, because of this variation. Those that survive will reproduce and pass these traits on to their offspring. In addition, these variations may give the species opportunities to enter and survive in new environments. Thus, sexual reproduction often gives a species more opportunities to survive because individuals are more varied.
While many organisms reproduce using asexual reproduction, some also have the ability to reproduce sexually. One of the simplest types of sexual reproduction is called conjugation. In this process, a connection between the cytoplasm of two unicellular organisms is formed and nuclear material is exchanged. While these simple organisms cannot really be considered males or females because they do not have testes or ovaries, respectively, there are different mating types. There are chromosomal and biochemical differences between the two mating types. Conjugation can occur in bacteria and some protists, such as algae and paramecia.
In more complex organisms that undergo sexual reproduction, individuals are usually either male or female; the sexes are usually separate. Males and females each have different types of gonads or sex organs in which the gametes develop. The males produce sperm in gonads called testes and the females produce eggs (also called ova) in gonads called the ovaries. The sperm are generally smaller than the egg and must swim through a liquid to get to the egg in order for fertilization to occur. In most cases, when sexual reproduction occurs, the sperm and eggs originate from separate parents; one male and the other female. However, there are some organisms that can produce both sperm and eggs. These individuals that have both testes and ovaries are called hermaphrodites. However, it is relatively rare that the sperm from a hermaphroditic individual can fertilize its own eggs.
There are different strategies for sexual reproduction. Depending on where an organism lives, fertilization can occur inside the female (internal fertilization) or outside (external fertilization). Both strategies have advantages and disadvantages.
Aquatic organisms, those that live in water, more commonly use external fertilization. The sperm and eggs are released into the water where they meet and fertilization occurs. This is an advantage because the water is a liquid through which the sperm can swim to the egg. In addition, the female does not usually invest time and energy in care of her young. However, there is a very good chance that sperm and egg might not meet, they may succumb to predation by other organisms, or the environmental conditions might not be sufficient for survival of the gametes. In order to deal with these problems, organisms that use external fertilization release very large numbers of sperm and eggs into the water, and they usually release them at approximately the same time. For example, oysters release millions of sperm and eggs into the water at around the same time. Fertilization occurs externally, and the female provides no care for the developing offspring.
Terrestrial organisms, those that live on land, usually use internal fertilization. The sperm and egg meet inside the body of the female. With internal fertilization, both egg and sperm are protected, and there is a much greater chance that the gametes will meet. Thus, although there are still large numbers of sperm, fewer eggs are needed. In addition, the female's body usually provides the proper environmental conditions for survival of the gametes, unlike the environment outside of the body. Like external fertilization, timing of release of the gametes must be correct. There is often only a short period of time that the egg can be fertilized by the sperm, and mating must take place within this time period. In humans, for example, the egg is only able to be fertilized for approximately 24 hours during each 28 day cycle. Internal fertilization requires some specific adaptations of the organisms that use it. For example, the female must provide a moist environment through which the sperm can swim to the egg, and there must be a specialized sex organ that places the sperm in relatively close proximity to the egg. After internal fertilization occurs, the zygote is either released from the body within some protective covering (i.e., a shell) or it remains within the body where development occurs.
Regardless of whether fertilization is internal or external, once it occurs, the unicellular zygote goes through a series of cell divisions. At a certain point, the cells begin to specialize and differentiate into the individual body parts of that organism. Eventually, as development continues, a fully formed individual results. Thus, the end product of sexual reproduction is a new individual.
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